Threshold circuit for tone-ringer or like load means

ABSTRACT

1. A THRESHOLD CIRCUIT FOR ENERGIZING FROM A D.C. POWER SUPPLY SOURCE A FIRST LOAD MEANS HAVING FIRST AND SECOND TERMINALS, SAID CIRCUIT COMPRISING A DUMMY LOAD SWITCHABLY INTERCONNECTED ACROSS THIRD AND FOURTH TERMINALS FOR CONNECTION TO SAID POWER SUPPLY SOURCE WHEN SAID LOAD MEANS IS DISCONNECTED THEREFROM, SAID DUMMY LOAD HAVING FIFTH AND SIXTH TERMINALS AND ADAPTED TO DRAW LESS CURRENT THAN SAID FIRST LOAD MEANS, WHEREBY THE POTENTIAL DIFFERENCE BETWEEN SAID THIRD AND FOURTH TERMINALS IS GREATER BY A VALUE V1 WITH SAID DUMMY LOAD OPERATIVELY CONNECTED THEREBETWEEN THAN WITH SAID FIRST LOAD MEANS SO CONNECTED FOR A GIVEN POWER SUPPLY IMPEDANCE, SAID FIFTH TERMINAL CONNECTED TO SAID FOURTH TERMINAL OF SAID POWER SUPPLY SOURCE AND SAID SIXTH TERMINAL CONNECTED THROUGH A FIRST CONTROLLABLE SWITCH MEANS TO THE CONTROL ELEMENT OF A SECOND CONTROLLABLE SWITCH MEANS, THE CONTROL ELEMENT OF SAID FIRST CONTROLLABLE SWITCH MEANS CONNECTED THROUGH SAID SECOND CONTROLLABLE SWITCH MEANS TO SAID THIRD TERMINAL, A FOURTH CONTROLLABLE SWITCH MEANS BETWEEN SAID FIRST TERMINAL OF SAID FIRST LOAD MEANS AND SAID THIRD TER MINAL, THE CONTROL ELEMENT OF SAID FOURTH CONTROLLABLE SWITCH MEANS CONNECTED TO THE CONTROL ELEMENT OF SAID FIRST CONTROLLABLE SWITCH MEANS AND THROUGH A THIRD CONTROLLABLE SWITC MEANS TO SAID FOURTH TERMINAL, VOLTAGE THRESHOLD MEANS COMPRISING FIRST AND SECOND SERIES CONNECTED PORTIONS THEREOF AND HAVING A THRESHOLD VALUE V2, CONNECTED BETWEEN SAID THIRD TERMINAL AND THE CONTROL ELEMENT OF SAID THIRD CONTROLLABLE SWITCH MEANS, SAID SECOND PORTION BEING CONNECTED TO SAID CONTROL ELEMENT OF SAID THIRD CONTROLLABLE SWITCH MEANS, AND UNIDIRECTIONAL CURRENT PASSING MEANS CONNECTING THE JUNCTION BETWEEN SAID FIRST AND SECOND PORTIONS OF SAID THRESHOLD MEANS TO SAID FIRST TERMINAL OF SAID FIRST LOAD MEANS AND ADAPTED TO BLOCK CURRENT FLOW BETWEEN SAID FIRST TERMINAL OF SAID FIRST LOAD MEANS AND SAID THIRD TERMINAL WHEN SAID FOURTH CONTROLLABLE SWITCH MEANS IS DISABLED, SAID SECOND TERMINAL OF SAID FIRST LOAD MEANS BEING CONNECTED TO SAID FOURTH TERMINAL, WHEREBY A CURRENT PATH IS ESTABLISHED BETWEEN SAID THIRD TERMINAL AND SAID CONTROL ELEMENT OF SAID THIRD CONTROLLABLE SWITCH MEANS ENABLING SAID THIRD CONTROLLABLE SWITCH MEANS WHEN THE POTENTIAL DIFFERENCE BETWEEN SAID THIRD AND FOURTH TERMINALS REACHES SAID THRESHOLD VALUE V2, THE BREAKDOWN VOLTAGES OF SAID UNDIRECTIONAL CURRENT PASSING MEANS AND SAID SECOND PORTION OF SAID THRESHOLD MEANS BEING SUCH AS TO PASS ENABLING CURRENT TO THE CONTROL ELEMENT OF SAID THIRD CONTROLLABLE SWITCH MEANS WHEN SAID FOURTH CONTROLLABLE SWITCH MEANS IS ENABLED AND THE POTENTIAL DIFFERENCE BETWEEN SAID THIRD AND FOURTH TERMINALS IS WITHIN THE RANGE OF VALUES FROM (V2-V1) OR LOWER TO V2.

Dec. 24, 1,74 J CQWPLAND ETAL Re. 28, 283

THRESHOLD CIRCUIT FOR TONE-RINGER OR LIKE LOAD MEANS Original FiledSept. 12, 1972 United States Patent 28,283 THRESHOLD CIRCUIT FORTONE-RINGER OR LIKE LOAD MEANS Michael C. J. Cowpland and Richard C.Foss, Ottawa,

Ontario, Canada, assignors to Microsystems International Limited,Montreal, Quebec, Canada Original No. 3,772,470, dated Nov. 13, 1973,Ser. No. 288,410, Sept. 12, 1972. Application for reissue Feb. 4, 1974,Ser. No. 439,065 Claims priority, application Canada, July 24, 1972,147,766 Int. Cl. H04m 1/00 US. Cl. 17984 T 11 Claims Matter enclosed inheavy brackets appears in the original patent but forms no part of thisreissue specification; matter printed in italics indicates the additionsmade by reissue.

ABSTRACT OF THE DISCLOSURE The present invention relates to a thresholdcircuit for connection between the terminals of a power-supply and aloadparticularly a tone-ringer for use in a station-set of a telephoneor like data-transmission system. The circuit is used to switch betweena dummy load and a main load-such as a tone-ringerwherein the dummy loadnormally takes lesser current draw than the main load. Therefore, for agiven power-supply impedance, the voltage drop across the power-supplylines will increase and in conventional circuits this reduces thevoltage level below the threshold, causing parasitic oscillation. Thepresent invention eliminates this problem by providing hysteresis in thethreshold voltage by use of a positive feedback network, so that thedummy load will not be switched in again until the power-supply voltageis considerably less than the original threshold voltage level.

The present invention relates to a threshold circuit for connectionbetween the terminals of a power supply and a loadparticularly atone-ringer for use in a station set of a telephone or likedata-transmission system.

Tone ringers for standard telephone systems must opcrate off the sameringing voltage as a conventional telephone bell. For North Americansystems, this voltage is typically 88 volts AC. at Hz., similar voltagesand frequencies being commonly employed in other systems throughout theworld. Two sources of spurious signals which can trigger a tone-ringerinto operation are switchhook transients and the pulses caused by rotarydials. The problem is particularly serious on short telephone loopswhere the inductance of the central office relays can cause largevoltage transients under pulsing conditions.

Therefore, some kind of threshold circuit is desirable to ensure thatthe tone-ringer will not respond to spurious signals. conventionally,the AC. power supply signal is full-wave rectified and the DC. outputfrom the rectifier stored in a capacitor. The threshold voltage of thecapacitor is selected to be higher than the voltage caused by spurioussignals on the telephone line, and is typically about 17 volts. Untilthis threshold value is reached, a dummy load is placed in circuitacross the capacitor to discharge any spurious charge thereon caused byrotary dial transients, etc. When the threshold voltage is reached, itis desirable to remove the dummy load completely to avoid excessivepower consumption through the lines. This is important from theviewpoint of maximizing the number of toneringers which can be poweredin parallel by the system. Also, the maximum number of paralleltone-ringers that can be used is constrained by the operating currentrequired by them and, hence, the power drain of the dummy loads alsoconstrain the maximum number which can be used. Thus, it is obviouslydesirable that the dummy load as well as the tone-ringer should draw aslittle current as possible and the dummy load clearly should not drawmore current than the tone-ringer.

Now, examine the case where the tone-ringer draws about 2.5 milliamps ofcurrent (typical) and the dummy load, say, 2 milliamps. When thethreshold voltage is reached for the tone-ringer, the tone-ringer isenergized and the dummy load is switched out of circuit. Now the currentdraw rises to 2.5 milliamps--a rise of 0.5 milliamps. Assuming a sourceimpedance (telephone line, etc.), of about 8 Kohms, this rise gives anextra voltage drop of 4 volts, reducing the voltage level below thethreshold and causing parasitic oscillation.

The present invention avoids this problem by arranging hysteresis in thethreshold voltage by use of a positive feedback network. Thus, when thedummy load switches over to the tone-ringer, the threshold level dropsso that the voltage drop due to the extra current draw by thetone-ringer still does not pull the operating voltage down to thethreshold level which would otherwise cause the dummy load to beswitched in again and thus give rise to the parasitic oscillationcondition.

According to the present invention there is provided a threshold circuitfor energizing from a DC. power supply source a first load means havingfirst and second terminals, said circuit comprising a dummy loadswitchably interconnected across third and fourth terminals forconnection to said power supply source when said load means isdisconnected therefrom, said dummy load having fifth and sixth terminalsand adapted to draw less current than said said first load means,whereby the potential difference between said third and fourth terminalsis greater by a value V with said dummy load operatively connectedtherebetween than with said first load means so connected for a givenpower supply impedance, said fifth terminal connected to said fourthterminal of said power supply source and said sixth terminal connectedthrough a first controllable switch means to the control element of asecond controllable switch means, the control element of said firstcontrollable switch means connected through said second controllableswitch means to said third terminal, a fourth controllable switch meansbetween said first terminal of said first load means and said thirdterminal, the control element of said fourth controllable switch meansconnected to the control element of said first controllable switch meansand through a third controllable switch means to said fourth terminal,voltage threshold means comprising first and second series connectedportions thereof and having a threshold value V connected between saidthird terminal and the control element of said third controllable switchmeans, said second portion being connected to said control element ofsaid third controllable switch means, and unidirectional current passingmeans connecting the junction between said first and second portions ofsaid threshold means to said first terminal of said first load means andsaid third terminal, current flow between said first terminal of saidfirst load means and said third terminal when said fourth controllableswitch means is disabled, said second terminal of said first load meansbeing connected to said fourth terminal, whereby a current path isestablished between said third terminal and said control element of saidthird controllable switch means enabling said third controllable switchmeans when the potential difierence between said third and fourthterminals reaches said threshold value V the breakdown voltages of saidunidirectional current passing means and said second portion of saidthreshold means being such as to pass enabling current to the controlelement of said third controllable switch means when said fourthcontrollable switch means is enabled and the potential differencebetween said third and fourth terminals 3 is within the range of valuesfrom (V V,) or lower to V In a preferred embodiment of the inventioneach of said controllable switch means is a bipolar transistor and saidunidirectional current passing means is a diode.

In yet a further embodiment of the invention, said first load means is atone-ringer.

The invention will now be described further by way of example only andwith reference to the accompanying drawings wherein:

FIG. 1 is a block diagram of a threshold circuit according to theinvention;

FIG. 2 is a schematic diagram of a threshold circuit according to afurther embodiment of the invention; and

FIG. 3 is a schematic diagram of a threshold circuit according to yet afurther embodiment of the invention.

Referring now to the drawings, and in particular to FIG. 1 thereof, thethreshold circuit comprises input terminals T, and T for connection to apower-supply. Although such power-supply is not shown in FIG. 1, itcommonly includes a capacitor across which terminals T, and T areconnected and which is charged by an input supply.

For the purposes of explanation, current is assumed to flow from T, to T(positive to negative), and the nomenclature applied to the circuitelements is formulated accordingly. Of course, if current flow isassumed to flow from T to T the following description is applicable withtransposition of nomenclature such as input" and output, etc. The load10 is connected at one side to terminal T and at the other side to theoutput terminal of a switch means Q The switch means Q, and each of theother switch means of the circuit may be a bipolar transistorwhich ispreferred-a field-effect transistor or a vacuum tube. The circuit willhave appropriate impedance matching means-and auxiliary power-supplymeans, where requiredfor the particular type of switch means selected,such requirements being readily ascertainable to those skilled in theart from a consideration of the basic circuit of the invention. Acrossthe terminals T, and T there is connected a dummy-load 12 which isseries connected with the terminals of a switch means Q, and thejunction between the control electrode and input terminal of a switchmeans Q this junction being denoted by the resistance R,. The outputterminal of Q, and the control electrode of Q, are interconnected andare also connected to the control electrode of Q, and to the inputterminal of a switch means Q the output terminal of which is connectedto T The control electrode of Q, is connected to the input terminal ofQ, through a threshold means 11. The threshold means 11 is partiallyby-passed by a unidirectional current passing means-shown as a diodeD,acrss the switch means 0,. The threshold means 11 preferably comprisesseries diodes-conveniently Zener diodes-to the junction between whichthe connection to diode D, is made.

The circuit operates as follows. When the voltage across terminals T,and T is less than the threshold voltage for the circuit, the thresholdmeans 11 prevents enabling potential from reaching the control electrodeof switch means Q Now, noise spikes at the control electrodes of Q, andQ, cause these switches to saturate and effectively function as athyristor, clamping the dummy load 12 to terminal T,, less the smallpotential drop across resistance R,. The diode D, is effective to blockcurrent flow from terminal T, through the threshold means 11 to the loadwhen Q, is disabled.

When the voltage between T, and T, reaches the threshold voltage, thethreshold means 11 conducts and applies an enabling potential to Q Nowpotential from the control electrode of Q, is drained through Qdisabling Q, and Q Switch means Q deriving enabling potential from thenegative terminal T through switch means Q, is now enabled, thusconnecting the load 10 across terminals T, T, and noise spikes at thebases of TR, and TR, quickly passed by current flowing through the diodeD thereby FIG. 2:

TR,, TR,, TR;,, TR, Base-emitter junction of T:

i, ZD2 s Also included in the circuit are impedance matching resistorsR, and R each connected to the collector of TR; and respectively to thebase of TR, and to the base of TR The diode D, is connected to thejunction of ZD, and R The operation of the circuit of FIG. 2 is entirelyanalogous to that of FIG. 1 and is as follows.

When the voltage across T, and T is less than the threshold voltage forthe circuit, the threshold means, constituted by ZD,, R and ZD; does notconduct, and insuflicient potential appears at the base of TR, to enableTR Therefore, the base of TR, is located from terminal T and noisespikes at the bases of TR, and TR quickly cause these transistors tosaturate. In this condition, resistor R is placed in circuit acrossterminals T, and T and the end thereof connected to the emitter of TR,is clamped to the potential of T, minus the diode drop across thebase-emitter junction of TR,,.

When the voltage rises above the threshold value for the circuit, ZD,and ZD, break down and the threshold means conducts, applying potentialto the base of TR,,, enabling TR Now, charge is drained from the base ofTR, to terminal T disabling TR, and TR, and elfectively disconnecting R;from the circuit. With TR; now conducting, and TR, disabled, charge fromterminal T is applied to the base of TR.,, enabling TR, and connectingthe load 10 across the terminals T, and T With TR, conducting, a currentpath is also established through D, bypassing ZD, and effectivelylowering the threshold of the voltage applied to the base of TR, by aZener diode drop ZD, minus the drop across diode D,. It should be notedin this connection that the primary purpose of D, is as a unidirectionalcurrent passing device but that it is usefully employed also as abreakdown device and for this purpose a Zener diode can. equally Well besubstituted.

Thus the effective threshold voltage for the circuit is reduced, givingthe required hysteresis. If the threshold voltage for the circuit is V,and the required hysteresis is V,, to prevent parasitic oscillation,transistor TR, must remain enabled over the range of voltages from(V,V,) to V Normally, the hysteresis is set to be slightly greater thanV, to provide for circuit parameter fluctuations and, therefore, diodeD, and Zener diode ZD, are set to break down at a level slightly lowerthan (V V,).

FIG. 3 shows a circuit according to a further embodiment of theinvention, particularly suitable for fabrication by integrated circuittechnology. The elements of FIG. 2 appear in FIG. 3 and are designatedby like numerals and lettering in both Figures.

The input to terminals T, and T is derived from an alternating currentsupplyfor example a telephone linethrough terminals T and T and afull-wave bridge rectifier. The full-wave rectifier comprises diodes Dto D inclusive and a storage capacitor C, is placed across terminals T,and T The charge on C, eifectively comprises the D.C. power supply forthe threshold circuit. A coupling capacitor C is placed in series withan impedance matching resistor R between terminal T and the junction ofdiodes D and D in order to: prevent DC. current drain on the line.

Instead of the base electrode of TR being directly connected to thecollector electrode of TR such connection is made through a diode DAlso, the collector electrode of TR is connected to the collectorelectrode of TR; through a diode D Diodes D and D prevent TR fromsaturating. If saturation were allowed to occur, parasitic PNP action tothe integrated circuit substrate could arise.

In place of single transistor TR, as shown in FIG. 2 the circuit of FIG.3 employs a multiple transistor arrangement designated generally as TR,and comprising transistors TR TR and TR Transistor TR, functions astransistor TR, in FIG. 2 and transistor TR acts as a buffer to providethe main power supply current to the load. The function of transistor TRwill be explained below.

Diode D is inserted between terminal T and the emitter of TR andfunctions to ensure that T R.,,, and T11 are disabled when TR is on butheld out of saturation by D5 and D7.

Zener diodes ZD and 2D, are inserted across resistor R and serve toprotect capacitor C from excessive voltages by clamping it to anappropriate maximum value.

Transistor TR is connected between the base electrode of TR and thecollector of TR; and functions as a buffer for TR Resistor R connectedbetween the base of TR and terminal T sinks leakage current through theZener diode train to terminal T The circuit comprising transistors TRTR; and TR and diode D functions to provide a low-voltage regulatedpower supply at terminal T which is particularly convenient foroperating various functions with which the circuit is associated. Forexample, if the circuit were used with an electronic tone-generator in atelephone or like data-transmission station-set, such low voltage powersupply would be useful for operating the tone-generating portions of thecircuit.

Various alternatives and modifications to the embodiments disclosedherein will be readily apparent to those skilled in the art withoutdeparting from the spirit and scope of the invention as described by thedisclosure and defined by the claims appended hereto.

What is claimed is:

1. A threshold circuit for energizing from a DC power supply source afirst load means having first and second terminals, said circuitcomprising a dummy load switchably interconnected across third andfourth terminals for connection to said power supply source when saidload means is disconnected therefrom, said dummy load having fifth andsixth terminals and adapted to draw less current than said first loadmeans, whereby the potential difference between said third and fourthterminals is greater by a value V with said dummy load operativelyconnected therebetween than with said first load means so connected fora given power supply impedance, said fifth terminal connected to saidfourth terminal of said power supply source and said sixth terminalconnected through a first controllable switch means to the controlelement of a second controllable switch means, the control element ofsaid first controllable switch means connected through said secondcontrollable switch means to said third terminal, a fourth controllableswitch means between said first terminal of said first load means andsaid third terminal, the control element of said fourth controllableswitch means connected to the control element of said first controllableswitch means and through a third controllable switch means to saidfourth terminal, voltage threshold means comprising first and secondseries connected portions thereof and having a threshold value Vconnected between said third terminal and the control element of saidthird controllable switch means, said second portion being connected tosaid control element of said third controllable switch means, andunidirectional current passing means connecting the junction betweensaid first and second portions of said threshold means to said firstterminal of said first load means and adapted to block current flowbetween said first terminal of said first load means and said thirdterminal when said fourth controllable switch means is disabled, saidsecond termnial of said first load means being connected to said fourthterminal, whereby a current path is established between said thirdterminal and said control element of said third controllable switchmeans enabling said third controllable switch means when the potentialdifference between said third and fourth terminals reaches saidthreshold value V the breakdown voltages of said unidirectional currentpassing means and said second portion of said threshold means being suchas to pass enabling current to the control element of said thirdcontrollable switch means when said fourth controllable switch means isenabled and the potential difference between said third and fourthterminals is within the range of values from (V -V or lower to V2.

2. The threshold circuit of claim 1 wherein each of said controllableswitch means is a bipolar transistor.

3. The threshold circuit of claim 1 wherein each of said first andsecond portions of said voltage threshold means includes a voltagebreakdown device comprising a diode means.

4. The threshold circuit of claim 3 wherein each of said diode means isa Zener diode.

5. The threshold circuit of claim 1 wherein said unidirectional currentpassing means is a diode.

6. A threshold circuit for energizing from a DC. power supply source afirst load means having first and second terminals, said circuitcomprising a dummy load switchably interconnected across third andfourth terminals for connection to said power supply source when saidload means is disconnected therefrom, said dummy load having fifth andsixth terminals and adapted to draw less current than said first loadmeans, whereby the potential difference between said third and fourthterminals is greater by a value V with said dummy load operativelyconnected thercbetween than with said first load means so connected fora given power supply impedance, said fifth terminal connected to saidfourth terminal of said power supply source and said sixth terminalconnected through a first transistor to the base electrode of a secondtransistor, the base electrode of said first transistor connectedthrough said second transistor to said third terminal, a fourthtransistor between said first terminal of said first load means and saidthird terminal, the base electrode of said fourth transistor connectedto the control element of said first transistor and through a thirdtransistor to said fourth terminal, voltage threshold means comprisingfirst and second series connected portions thereof and having athreshold value V each of said first and second series connectedportions including a Zener diode connected be tween said third terminaland the base electrode of said third transistor, said second portionbeing connected to said base electrode of said third transistor, anddiode means connecting the junction between said first and secondportions of said threshold means to said first terminal of said firstload means and adapted to block current flow between said first terminalof said first load means and said third terminal when said fourthtransistor is disabled, said second terminal of said first load meansbeing connected to said fourth terminal, whereby a current path isestablished between said third terminal and said base electrode of saidthird transistor enabling said third transistor when the potentialdifference between said third and fourth terminals reaches saidthreshold value V the breakdown voltages of said diode means and saidsecond portion of said threshold means being such as to pass enablingcurrent to the base electrode of said third transistor when said fourthtransistor is enabled and the potential difference between said thirdand fourth terminals is within the range of values from (V V or lower toV2.

7. The threshold circuit of claim 6 wherein said first load meanscomprises a tone-ringer for a station-set and said DC. power supplysource is derived through rectifying means from an alternative currentenergized telephone or data-transmission line.

8. A threshold circuit for energizing a first load from a direct currentpower supply source of varying voltage comprising:

(a) means connected across said source, and to a dummy load, forenergizing the dummy load at power supply voltages below a predeterminedthreshold level,

(b) switch means connected to said source and to said first load forconnecting said first load to said source when activated, at powersupply voltages above said predetermined threshold level,

(c) means within the (a) means for simultaneously disconnecting saiddummy load and, at the same time, activating said switch means, at powersupply voltages above said predetermined threshold level, and

(d) means connected between the (a) means and the switch means forreducing the voltage of said threshold level upon actuation of saidswitch means.

9. A threshold circuit as defined in claim 8, in which the (d) meanscomprises a nonlinear network connected to said source adapted to drawcurrent and activate the (c) means at power supply voltages in excess ofthe predetermined threshold level, and to draw current and continue toactivate the (c) means at reduced power supply voltages once the switchmeans has operated.

10. A threshold circuit as defined in claim 9 in which the nonlinearnetwork comprises a pair of series connected Zener diodes which has acombined conduction voltage substantially equal to said predeterminedthreshold level,

one of said Zener diodes having a first terminal connected to a supplyterminal and a second terminal connected to the other of said Zenerdiodes; and in which said switch means comprises the emitter-collectorcircuit of a transistor activatable by its emitter, connected betweenthe power supply terminal to which said first terminal of said one ofthe Zener diodes is connected and the first load, further comprising adiode connected between the collector terminal of the transistor andsaid second terminal of said one Zener diode, whereby the conductionvoltage of the Zener diode which is connected to the (c) means forms thereduced said threshold level.

11. A threshold circuit as defined in claim 8, said first loadcomprising a ringing ocillator, further comprising means interconnectingthe (a) and (b) means adapted to disconnect the ringing oscillator fromacross said line in the event the line voltage drops below a secondpredetermined level lower than the first predetermined level, at thetime causing the dummy load to be reconnected across said line.

References Cited The following references, cited by the Examiner, are ofrecord in the patented file of this patent or the original patent.

UNITED STATES PATENTS 3,731,004 5/1973 Cowpland 17984 T 3,291,91612/1966 Jorgensen 17984 T FOREIGN PATENTS 1,228,672 11/1966 Germany17984 T KATHLEEN H. CLAFFY, Primary Examiner J. A. POPEK, AssistantExaminer

